The disclosure of Japanese Patent Application No. 2002-050331 filed on Feb. 26, 2002 including the specification, drawings and abstract are incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to an automatic transmission that is mounted in a vehicle such as an automobile. More specifically, the present invention relates to a structure of a gear section that is designed for use in an automatic transmission provided with a plurality of clutches disposed together in a front half portion, with the gear section in a rear half portion.
2. Description of Related Art
An automatic transmission that is mounted in a vehicle such as an automobile and that is provided with a plurality of clutches disposed together in a front half portion, with gearing in a rear half portion is conventional. FIGS. 11 and 12 show known examples of this kind of automatic transmission. The automatic transmissions shown in these skeletal drawings are disclosed, in the order shown, as FIGS. 1 and 3 of Japanese Patent Laid-Open Publication 04-285347. These automatic transmissions are provided with clutches C1 and C2 disposed together in a front half portion and gearing (first, second and third planetary gear sets G1, G2 and G3) in a rear half portion, and provide six forward shift speeds by use of the gear trains shown in FIGS. 11 and 12. Note that Explanation of symbols that is unnecessary for understanding this related art has been omitted where appropriate.
Recently, from the point of view of improving fuel consumption, there have been strong demands to make automatic transmissions multi-speed. At the same time, from the point of view of mountability, strong demands have been made to make the automatic transmission unit itself more compact. The objective of making automatic transmissions multi-speed, however, is at odds with the objective of making them more compact. Furthermore, it is also required that automatic transmissions have good controllability. In other words, for example, comfortable shifting without shift shock is required along with simple control. In general, as one method to achieve this, widespread use has been made of a one-way clutch.
In the automatic transmissions shown in FIGS. 11 and 12, however, a one-way clutch is not provided and, therefore it can be inferred that shifting is carried out using a clutch-to-clutch method which incurs the problem of poor controllability.
Here, for example, if an attempt were to be made to provide six forward speeds by use of a one-way clutch as an essential element of the structure of the related art described above, one approach might be modification of the structures shown in FIGS. 11 and 12 by providing the one-way clutch in parallel with a third brake B3, in relation to a rotating element in the form of the sun gear S1 in FIG. 11 or the carrier PC1 in FIG. 12. However, providing a one-way clutch in such a relationship to the rotating element would have the one-way clutch stop reverse rotation of the rotating element because the input shaft I and the rotating element are directly linked. It would therefore be impossible to achieve six speeds.
In view of the foregoing problems, it is an object of the present invention to provide an automatic transmission that has a compact structure and excellent controllability with one-way clutch shifting.
Accordingly, the present invention provides an automatic transmission including an input shaft, a first planetary gear unit, a second planetary gear unit or xe2x80x9csetxe2x80x9d (a rear gear unit) that includes at least four rotating elements, and an output shaft. The automatic transmission changes rotational speed of the input shaft among a plurality of shift speeds and outputs the changed speed at an output shaft by use of the planetary gear units. In the automatic transmission, the first planetary unit includes a first speed-changing element (a sun gear) to which the rotation of the input shaft is input, a second speed-changing element (a carrier) that is engaged and disengaged by a first engagement mechanism (a first brake) and a third engagement mechanism (a third brake and a first one-way clutch), and a third speed-changing element (a ring gear) that transfers rotation, at a different speed than that of the rotation of the input shaft, to the second (rear) planetary gear unit (xe2x80x9csetxe2x80x9d). The first engagement mechanism is in the form of the first brake, and the third mechanism includes the first one-way clutch, which engages and disengages the second speed-changing element, and the third brake which engages and disengages the first one-way clutch.
According to the first aspect of the present invention, because the first one-way clutch engages and disengages both the second speed-changing element of the first planetary gear unit and the third brake, reverse rotation of the second speed-changing element is made possible by disengaging the first and third brakes. Such an arrangement enables, for example, six forward speed shifting. In addition, since one-way clutch shifting can be carried out using the first one-way clutch, a simple structure is achieved and controllability is improved.
Optionally, the first speed-changing element may be connected to a second one-way clutch, and the rotation of the input shaft transferred through a third clutch. With the first speed-changing element arranged in such a manner, reverse rotation is made possible by use of the second one-way clutch and the third clutch.
The first planetary gear unit may be a double-pinion planetary gear unit, wherein the first speed-changing element is a sun gear, the second speed-changing element is a carrier, the third speed-changing element is a ring gear, and at least 6 forward speed shift speeds can be achieved by the combination of the first and second planetary gear units.
Preferably, the first brake is arranged radially outward of the first planetary gear unit, the third brake is provided on the front side of and substantially axially aligned with the first brake, a third brake hydraulic actuator is provided on the front side of and axially aligned with the third brake, the first one-way clutch is provided on the front side of the first planetary gear unit, the second one-way clutch is provided on the front side of the first one-way clutch, and the first and second one-way clutches are positioned radially inward of the third brake and the third brake hydraulic actuator.
With the above-described structure spatial relationships of the components can be optimized. The first brake, the third brake and the third brake hydraulic actuator are substantially axially aligned in sequence from the rear side, radially outward of the first planetary gear unit. The first planetary gear unit, the first one-way clutch and the second one-way clutch are axially aligned in sequence from the rear side, radially inward of the first and third brakes and third brake actuator. In other words, the first and third brakes, the second brake hydraulic actuator, the first planetary gear unit, and the first and second one-way clutches can be provided with good spatial efficiency, both radially and axially, thus making the automatic transmission more compact.
Thus, the automatic transmission according to the present invention may be structured such that, in the first forward speed stage beyond (for example 5th speed) a directly connected stage (for example 4th speed), the first brake is engaged and the carrier of the first planetary gear unit is thereby stopped, and, upon shifting to a second higher speed stage (for example 6th speed) above the directly connected stage, the first and third brakes are disengaged, and the second one-way clutch is freed and permits reverse rotation of the carrier of the first planetary gear unit.
Therefore, the first one-way clutch may be arranged so as to be connected to the transmission case via the third brake, instead of being directly connected to the transmission case. The first one-way clutch, therefore, can function as a one-way clutch when the third brake is engaged, i.e. permitting forward rotation of the first planetary gear unit carrier and preventing reverse rotation of the same. At the same time, since the first one-way clutch is in a free state when the third brake is disengaged (a state identical to that where no one-way clutch is present), the first planetary gear unit carrier is permitted to rotate in either the forward or reverse direction and six forward speeds are enabled based on the enabling of reverse rotation of the first planetary gear unit carrier.